High mechanical strength or good creep characteristics at high usage temperatures are expected primarily from refractory metals as high temperature materials. In the past, there has been no lack of proposals to improve these properties via the formation of a alloys.
For example, an alloy is described in AT 386 612 (which corresponds to U.S. Pat. No. 4,950,327) which comprises high melting metals like molybdenum and tungsten with the formation of a specific structure which contains 0.005-10 wt % of one or more high melting compounds comprising oxides and/or silicates, among others. Molybdenum/tungsten alloys with small proportions of oxide, preferably below 2%, find use in practice.
Structural units which come into contact with glass melts and/or ceramic melts such as melt electrodes, linings, stirring devices, piping, returns and the like have to fulfill a number of stringent requirements simultaneously. In particular, they have to possess high chemical resistance relative to the melt, high resistance to oxidation, adequate mechanical strength at usage temperatures between approximately 1100.degree. C. and 1600.degree. C. as well as high mechanical resistance to changing temperatures. The requirement of good electrical conductivity also applies in the case of glass melt electrodes which are an example of such structural units.
In addition, the structural units should be readily processable by mechanical means and they should contaminate the melt as little as is possible.
In the past, the high melting metals molybdenum and tungsten and certain alloys of these metals, their silicides and also materials such as graphite, platinum or ceramic materials such as tin oxide have been used as a material for the manufacture of melt electrodes which constituted the largest proportion of such structural units from a quantity standpoint. In addition, attempts have also been made to improve their corrosion and oxidation characteristics further by means of special coatings of the electrode material.
Despite certain disadvantages, the high melting materials pure molybdenum, Mo30W or molybdenum with 0.5 vol % ZrO.sub.2 (known by the abbreviation PSZ, Z6) are used in practice as a material for structural units which come into contact with glass melts or ceramic melts using standard compositions (no special glass). However, the corrosion characteristics of these materials are unsatisfactory for strongly corrosive types of glass or ceramic melts. Strongly corrosive glass or ceramic melts are particularly those with appreciable proportions of polyvalent ions, e.g., As, Sb, Zn, Ni, Mn, Co, Pb and sulfate. The working life of the material is determined in this regard both by the uniform surface oxidative removal of molybdenum and by locally selective corrosive attack.
Until now, no economically and technically satisfactory procedures have become known which prevent the chemical contamination of highly corrosive glass melts, i.e., melts with additions which form polyvalent ions, via the products of the corrosion of the materials of the structural unit, i.e., molybdenum/tungsten. An aggravating aspect in addition is that the ongoing corrosion mechanisms, in the melt are not known in detail.
AT 386 843 describes the use of a heat resistant molybdenum alloy, which essentially comprises 0.05-19.9 wt % silicon with the remainder being molybdenum, for molded units, etc., which come into contact with ceramic melts. Such molded units have good oxidation and corrosion resistance in addition to excellent creep resistance at high usage temperatures. A disadvantage as a consequence of the high silicon content is that the designated alloy is practically incapable of being shaped mechanically so that the majority of structural units for glass melt devices cannot be economically manufactured.
U.S. Pat. No. 4,668,262 describes a structural unit, especially a glass melt electrode, of a high melting metal, preferably molybdenum or tungsten, which has been provided with an external protective layer of chromium oxide and an intermediate coat of molybdenum silicide in order to improve its oxidation resistance at high usage temperatures. Such glass melt electrodes have superb resistance to oxidation at high temperatures. However, a disadvantage is that the protective coat is degraded in the course of time and it therefore results in only very temporary protection against the oxidizing components of the melt. In addition, the dyeing of the glass melt by Cr oxide is incompatible in most cases. The long-term corrosion properties are not therefore improved by such protective coats.
Thus the problem which the present invention seeks to solve comprises the provision of an alloy for use in structural units which come into contact with glass and/or ceramic melts and which have improved corrosion resistance compared to known materials, especially with respect to highly corrosive glass and ceramic melts with additions of metal compounds forming polyvalent ions. In this connection, the improvement must not impair in any substantial manner either the economics of the previously used glass melt process or the other properties which are required of such materials for structural units.